Is this correct

Discussion in 'General Electronics Chat' started by sirch2, Feb 23, 2014.

  1. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
    (Apologies for the short title but couldn't post with a longer one)

    I have been looking at options for a thermistor based anemometer (see some of my other threads if interested). There is a company - Modern Devices - selling one which apparently uses the circuit below. I have one of these and was looking at the circuit when I noticed what I think is an error...

    I assume that the 10K Therm is there to balance out changes in ambient temperature and that the other thermistor is the one that reads the air flow. But looking closely it is not in the bridge created by R5/R6 - R3/Therm_0402. It is simply connected between supply and ground via R1

    Am I missing something?

  2. #12


    Nov 30, 2010
    Look at pin 5 of U1B for the connection marked, "TEMP_V"
  3. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
    Thanks for taking a look, so 10K_Therm is just a separate temp sensor and not there to correct the bridge for ambient temp.

    Useful but not what I was hoping for.
  4. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    A lot of these air flow units contain two thermistors in a bridge configuration and close proximity. One is shielded from air flow and senses ambient temp. The other, exposed to air flow looses heat to the air stream which unbalances the bridge in proportion to the speed of the air flow. My truck uses a system like that to calculate mass air flow into the turbo.
    Austin Clark likes this.
  5. sirch2

    Thread Starter Well-Known Member

    Jan 21, 2013
    Yes, that is what I was expecting...
  6. pilko

    Senior Member

    Dec 8, 2008
    This unit does not operate that way.
    Both thermistors are in the air flow. Thermistor R4 self heats through the 1 ohm resistor , it is then able to sense the air speed due to cooling effect. Its temperature/resistance is proportional to the air speed. Thermistor R2 is not self heated and therefore measures the air temperature for compensation.